The present invention relates to high frequency power supplies for use with luminous, e.g. neon, tubular glass signage of the type often found in connection with retail advertising and decorating. As outlined hereinafter, the present supply overcomes several problems endemic to this class of luminous tube power sources and, importantly, does so in a most efficacious, reliable, and cost effective manner. In this latter connection it will be appreciated that luminous tube supplies are used in large quantities and consequently any per-unit cost savings will have a profound impact on commercial viability and product profitability.
In the first instance, the present supply is generally of the non-resonant, fixed frequency variety. It is well known that the operating frequency of conventional resonant and similar free-running power supplies may vary dramatically as a function of luminous tube load (i.e. tube length) which, in turn, can result in decreased efficiency, supply non-starting, and an audible acoustic whine. Examples of known self-oscillating, free-running luminous tube power supplies includes U.S. Pat. Nos. 4,613,934 and 4,698,741.
Further, the transformer secondary windings required to generate the requisite luminous tube high voltage characteristically exhibit self resonances that fall close to, or within, the normal supply operating frequency range. Erratic and unpredictable supply performance can be expected where the supply is operated too close to such resonances. Thus, the present supply avoids these resonance-induced irregularities through the selection of an appropriate operating frequency--a frequency that remains substantially constant under all anticipated load conditions.
Although constant frequency luminous tube supplies are not new, known implementations have sacrificed both power (i.e. efficiency) and complexity (i.e. cost) to achieve the desired benefits of constant frequency operation.
Typically such supplies have employed a variable pulse width modulation (PWM) scheme in which the supply output current is regulated by varying the duration of a current pulse through the transformer primary winding. These current pulses are in turn gated by a PWM controller often of the integrated circuit variety.
Although PWM overcomes certain of the previously described problems of variable frequency, free-running supplies, conventional PWM systems have required significant circuitry including error amplifiers, ramp generators, flip-flop memory elements and voltage regulators. These elements all require electrical power. The Unitrode UC3843 PWM integrated circuit, for example, requires between 15-25 milliamperes at DC operating voltages of between 10-20 volts.
It is not this higher current, alone, that makes conventional PWM inefficient. Rather, it is the absence of a relatively low voltage DC supply to operate the PWM circuitry that presents the difficulty. In this connection, it will be noted that ordinary integrated circuits typically operate from a low voltage supply typically between 3-30 volts. The only and ultimate source of energy for luminous tube supplies is the 120 volt AC mains to which the supply is connected.
Several techniques for generating this low voltage are known including the incorporation of (1) a separate low voltage transformer, rectifier and regulator; (2) adding a third low voltage winding to the high voltage transformer; or, (3) a down-converter from the higher voltages available from the input line. Each of these solutions have their corresponding problems. Adding a winding to a transformer adds costs. Further, the PWM circuitry requires voltage which, in turn, is generated by the PWM circuit. In short, a start-up mechanism or voltage source must be provided.
Adding a separate low voltage transformer and supply is both bulky and, most importantly, expensive. And the final alternative, down converting or regulating from the line, requires either complicated and expensive switching convertors or series-pass regulation--the latter dissipating substantial amounts of unused energy in view of the PWM integrated circuit power requirements.
The present supply employs a unique "uniform pulse width" pulse width modulator in which substantially the only circuitry required is a constant frequency uniform pulse width generator or oscillator. In this connection any number of low current solutions are available including the extremely low power CMOS version of the ubiquitous 555 integrated timer. The power requirements of this device are so low that the very simple and economical series resistance, shunt zener style regulator performs admirably and without significantly lowering the overall efficiency of the luminous tube supply.
The 555 generates a periodic and constant stream of narrow pulses which, in turn, are coupled to the gate of, thereby switching "on", a power switching FET. More specifically, the 555 pulses, although of narrow width, are sufficient to charge the FET gate capacitance thereby assuring continued FET conduction after pulse cessation. The modulation of the pulse width, as required to facilitate output current regulation, is achieved through a current sense/compensation network which rapidly discharges the gate capacitance upon reaching the desired current/voltage point. In this manner a highly reliable, while elegant in its simplicity and low cost, luminous tube supply has been developed.
The advantages of and problems overcome by this supply, however, are not limited to those set forth above. For example, another problem associated with luminous tube power supplies intended to accommodate varying sign configurations is that of proper illumination intensity.
It is well known that the intensity of a luminous sign is generally related to its average gas current therethrough and, further, that the voltage required across the tube to generate such current is directly proportional to tube length. It will be appreciated that signs come in a variety of overall sizes and design complexities and consequently the amount, i.e. length, of luminous tube required will correspondingly vary from one application to another.
It is an objective of the present invention to provide, for each model power supply, the greatest range and flexibility with respect to the luminous tubes lengths that can be accommodated thereby to achieve the further economic advantages of quantity production through the minimization of inventory costs associated with stocking multiple components at the OEM part acquisition level and multiple models at the distribution level.
In this connection, one problem associated with conventional current mode regulated high voltage supplies, particularly of the constant frequency variety, is the observable decrease in tube illumination intensity as shorter tube lengths are adopted. This phenomenon has been traced to a corresponding decrease in average tube current--the average current required to effect full and proper illumination being generally constant and independent of overall tube length. It is the operating voltage across the tube that varies according to tube length.
The luminous supply of the present invention provides a substantially uniform average current without regard to the length of luminous tube utilized thereby facilitating adoption of a single model supply suitable for all normal sign configurations.
Although conventional current mode power supplies are regulated, the mode of regulation, as the name implies, is peak current regulation. Typically the high voltage transformer primary current is sampled with the width of each pulse being adjusted such that a predetermined peak current results.
However, as progressively shorter tubes are connected to such supplies, correspondingly lower load impedances, in particular inductances, are reflected back to the transformer primary which, in turn, causes the primary current to reach its predetermined trigger level more quickly. Thus, although the same maximum tube current is achieved, the average current is seen to decrease as a function of shortened tube length.
This problem has been virtually eliminated in the present supply through the use of an inexpensive but effective resistor/capacitor load current compensator. Importantly, this network, although operating at a substantially constant frequency independent of tube length, nevertheless serves to equalize the area under the respective current envelopes thereby forcing corresponding equal average tube currents. In this manner uniform tube illumination without regard to tube length is achieved.
Yet another problem encountered in luminous tube signage relates to the use of differing tube gases. Although neon is commonly employed in such signs, it will be appreciated that other gases, most notably mercury, are frequently employed where differing tube colors are required. Neon, for example, is known to produce the warmer tones including shades of red, orange, pink, and purple while mercury is preferred for the cooler spectral colors of blue, turquoise, white, or yellow. Mercury is particularly suited to coloration through the use of phosphors on the tubular glass envelop.
As detailed hereinafter, the use of certain gases, in particular mercury, in luminous signage creates special problems for which the present power supply is particularly adapted to solve. One such problem is the blackening of the tube ends, i.e. adjacent the electrode, after sustained luminous tube operation. The problem has become particularly acute with the recent substitution of high frequency power supplies for the conventional 60 Hz power transformer.
In this connection it has been discovered that the application of an asymmetrical waveform to a mercury luminous tube--a not-uncommon occurrence with conventional high frequency luminous tube power supplies--results in a cataphoresis effect whereby positive ions are seen to migrate in a correspondingly asymmetric manner.
Mercury and neon differ in one important respect--mercury has a significantly higher vaporization temperature which permits mercury to remain in the liquid state under ordinary room temperature conditions. Thus, unlike neon, where normal Brownian motion assures the migration of neutralized gas ions thereby assuring substantially uniform gas distribution throughout the tubular glass envelope, mercury can condense on the envelope--discoloring the envelop and depleting the uniform distribution and availability of mercury gas molecules throughout the tube.
It has been determined that the above-described deleterious effects of mercury-filled luminous tubes can be alleviated by averaging, on a direct current basis, the waveform asymmetry even though the resulting waveforms retain their overall non-symmetrical character. To this end, capacitance is placed in the power supply output which, as presently understood balances the output waveform but, in any event, has been found to dramatically reduce the long-experienced problem of mercury tube blackening.
Yet another feature of the present invention is its inexpensive, yet improved, ground fault safety system. Ground fault detectors have become an important and mandated tool for the minimization of shock or electrocution occasioned by the inadvertent contact with electrical circuitry, in the present case, luminous tube signage. Ground fault detectors seek to measure and limit `unauthorized` currents to ground. Such currents are considered to be `unauthorized` in the sense that ground currents should not exist under normal equipment operating conditions and, further, that the mostly likely path for a lethal current would be to ground.
Ground fault detection operates on the principle of measuring any imbalance between the respective power source lines--any inequality therebetween defining an otherwise unaccounted for `missing` or ground fault current. Ground fault detectors are not new to the luminous tube power supply field, for example, U.S. Pat. No. 4,613,934. The present arrangement, however, provides for improved and more accurate ground fault detection, all for lower cost.
The detector described in the above-noted '934 patent employs the well-known method illustrated in FIG. 4 in which a current transformer is placed in the ground return path from the center-tap of the high voltage transformer secondary. In the absence of any unscheduled ground fault currents, the secondary winding current will be balanced with negligible current through the center-tap and current transformer. Should a ground fault condition exist, however, the '934 patent describes a single peak detector that triggers a ground fault alert/shut-down upon a current excursion exceeding a predetermined maximum safe limit. The '934 is sensitive, however, only to single polarity current excursions.
The present ground fault detector does not require, in the first instance, a specially wound, center-tapped transformer. In this connection it should be noted that the requirement for an additional tap in any high voltage winding requires special care to avoid inter-winding and winding-to-core shorts. Center-tapped transformer are correspondingly more expensive. Rather, the present ground fault detector employs capacitive center-tapping. Such center-tapping, however, is achieved through the use of the intrinsic secondary intra-winding capacitances, in particular, the distributed winding capacitances to the transformer core. By winding a symmetric secondary (i.e. with respect to the core), the core itself becomes the capacitive center, or center-tap, of the transformer thereby obviating any need, not only for the previously noted inductance winding center-tap, but for external capacitors as well.
As discussed, conventional luminous tube ground fault detectors such as disclosed in the '934 patent employ a single polarity peak current detector arrangement--this upon the faulty assumption that such currents are symmetrical. Although ground fault currents are AC, it has been observed that such currents are seldom symmetrical. Thus, the corresponding positive and negative peak amplitudes are rarely equal, sometimes differing by a factor of five to one. The difficulty associated with the unipolarity detection arrangement of the '934 patent is (1) the varying ground fault sensitivity from one ostensibly identical unit to another; (2) the inability to obtain repeatable ground fault interruption by any given unit under successively induced faults of constant magnitude; and, (3) the varying ground fault sensitivity from one supply lead compared to the other.
The above problems have been significantly reduced or eliminated in the present luminous tube supply through the use of a dual peak detector in which both positive and negative ground fault current peaks are detected and summed to provide a composite detection voltage. In this manner variations between respective polarity peaks are neutralized with the resultant detected ground fault signal being closely and repeatably related to the actual exigent ground fault current.
Other advantages and objects of the present invention in addition to those already discussed are set forth in, or will become apparent from, the drawings and the detailed description of the invention herein.